There are multiple reasons to raise preflop, I will try to cover some

1. Raise to isolate - a lot of hands play better vs one or two opponents then multiple opponents (AA/KK) is the classic example, because you rely on the strength of your pocket pair and with multiple opponents there is a good chance someone will improve to 2 pair or better after the flop

2. Raise for value - you have a good hand and you want to have a big pot

3. Raise to pick up the blinds

4. Raise to take control of the pot, as a general rule of thumb people respect raises, so often when you raise preflop you can pick up the pot after the flop when opp misses even if you miss your flops

5. Raise as part of a balanced game strategy (if you only raise your big hands you will not get action when you have them)